1. Field of the Invention
The present invention relates to an optical shifter that can be used to physically shift, or displace, the optical axis of outgoing light such that an optical output signal being presented on the screen moves to another location in a head mounted display (HMD) or a projection type optical display system (i.e., projector) and also relates to a method for driving such an optical shifter. The present invention further relates to a projection type optical display system including such an optical shifter.
2. Description of the Related Art
A projection type optical display system that uses a liquid crystal display (LCD) panel is known as an optical display system. Such a projection type optical display system needs to be separately provided with a light source because the LCD panel itself emits no light. However, the projection type optical display system using an LCD panel is advantageous over a projection type optical display system using a CRT, because the display system of the former type realizes a broader color reproducible range, has a smaller size and a lighter weight, and needs no convergence correction.
A projection type optical display system may conduct a full-color display operation either by a three-panel method (i.e., with three LCD panels used for the three primary colors) or by a single-panel method (i.e., with just one LCD panel used).
A three-panel projection type optical display system uses an optical system for splitting white light into three light rays representing the three primary colors of red (R), green (G) and blue (B) and three LCD panels for modulating the R, G and B light rays and thereby forming three image components. By optically superimposing the R, G and B image components one upon the other, the three-panel projection type optical display system can create an image in full colors.
The three-panel projection type optical display system can efficiently utilize the light that is radiated from a white light source but needs a complicated optical system and a greater number of components. Thus, the three-panel projection type optical display system is normally less advantageous than the single-panel projection type optical display system in respects of cost and size.
The single-panel projection type optical display system uses a single LCD panel including multiple R, G and B color filters that are arranged in a mosaic or striped pattern, and gets a full-color image, displayed on the LCD panel, projected onto a projection plane (e.g., a screen) by a projection optical system. Such a single-panel projection type optical display system is described in Japanese Laid-Open Publication No. 59-230383, for example. The single-panel type uses only one LCD panel, and needs an optical system that is much simpler than that of the three-panel type. Thus, the single-panel method can be used effectively to provide a small-sized projection type optical display system at a reduced cost.
In the single-panel type that uses color filters, however, light is absorbed into the color filters. Accordingly, compared to a three-panel type that uses a similar light source, the brightness of the image decreases to about one-third in the single-panel type. In addition, one pixel should be displayed by a set of three pixel regions of the LCD panel that correspond to R, G and B, respectively. Thus, the resolution of the image also decreases to one-third as compared to the three-panel type.
One of possible measures against that decrease in brightness is using a brighter light source. However, the use of a light source with great power dissipation for a consumer electronic appliance is not preferred. Also, when color filters of absorption type are used, the light that has been absorbed into the color filters changes into heat. Accordingly, if the brightness of the light source is increased excessively, then not only the temperature of the LCD panel increases but also the discoloration of the color filters is accelerated. For that reason, to increase the utility value of the projection type optical display system, it is very important how to make full use of the given light.
To increase the brightness of an image displayed by a single-panel projection type optical display system, a liquid crystal display device for conducting a display operation in full colors without using any color filter was developed and disclosed in Japanese Laid-Open Publication No. 4-60538, for example. In this liquid crystal display device, the white light that has been radiated from a light source is split into R, G and B light rays by dielectric mirrors such as dichroic mirrors. The light rays are then incident onto a microlens array at mutually different angles. The microlens array is provided on one side of an LCD panel so as to face the light source. These light rays that have been incident onto a microlens are transmitted through the microlens so as to be focused onto their associated pixel regions in accordance with the respective angles of incidence. Thus, the R, G and B split light rays are modulated by mutually different pixel regions and then used for a full-color display.
A display system, which uses transmissive hologram elements for the R, G and B light rays instead of the dielectric mirrors to utilize the light as efficiently as possible, is disclosed in Japanese Laid-Open Publication No. 5-249318. On the other hand, a display system, which includes a transmissive hologram element having a periodic structure defined by a pixel pitch and functioning as the dielectric mirrors or microlenses, is disclosed in Japanese Laid-Open Publication No. 6-222361.
The low resolution is another problem of the single-panel type. As for this problem, however, by adopting a field sequential technique, even just one LCD panel can achieve a resolution comparable to that of the three-panel type. The field sequential technique utilizes the phenomenon that when the colors of a light source are switched at too high a rate to be sensed by the human eyes, respective image components to be displayed time-sequentially have their colors mixed together by an additive color mixture process. This phenomenon is called a “continuous additive color mixture process”.
A projection type optical display system for conducting a full-color display operation by the field sequential technique may have a configuration such as that shown in FIG. 28, for example. In this optical display system, a disk, made up of R, G and B color filters, is rotated at a high velocity that corresponds to one vertical scan period of an LCD panel, and image signals, representing the colors of the three color filters, are sequentially input to the driver circuit of the LCD panel. In this manner, a synthesized image of three image components corresponding to the respective colors is recognized by human eyes.
In the display system of such a field sequential type, the R, G and B image components are displayed time-sequentially by each pixel of the LCD panel unlike the single-panel type. Thus, the resolution thereof is comparable to that of the three-panel type.
A projection type optical display system that irradiates mutually different regions of an LCD panel with the R, G and B light rays is disclosed as another display system of the field sequential type in Proc. International Display Workshop 1999 (IDW '99), December 1999, pp. 989–992. In this display system, the white light that has been radiated from a light source is split by dielectric mirrors into R, G and B light rays, which will be then focused onto mutually different regions of the LCD panel. The portions of the LCD panel to be irradiated with the R, G and B light rays are sequentially switched by rotating a cubic prism.
However, the display systems disclosed in Japanese Laid-Open Publications Nos. 4-60538, 5-249318 and 6-222361 identified above can increase the brightness but the resolution thereof remains one-third of that of the three-panel type. The reason is that three spatially separated R, G and B pixels are used as a set to represent one pixel (or dot).
In contrast, the normal field-sequential type can increase the resolution to a level comparable to that of the three-panel type. However, the brightness of the image achieved by the normal field-sequential type is no more satisfactory than the conventional single-panel type because the field-sequential type uses color filters.
In the display system disclosed in IDW '99 on the other hand, the points of incidence of the R, G and B light rays should not overlap with each other. For that purpose, illuminated light having a very high degree of parallelism is needed. Accordingly, the optical efficiency also decreases as being constrained by the degree of parallelism of the illuminated light.
Thus, none of the conventional techniques described above can increase the brightness and the resolution at the same time or solve the problems of the single-panel type.
To overcome these problems, the applicant of the present application proposed improved single-panel projection type optical display systems in Japanese Laid-Open Publication No. 9-214997 and in pamphlet of PCT International Publication No. WO 01/96932.
The projection type optical display system as disclosed in Japanese Laid-Open Publication No. 9-214997 uses a liquid crystal display device similar to that disclosed in Japanese Laid-Open Publication No. 4-60538 identified above. The display system also splits the white light into light rays in respective colors and then makes these light rays incident onto their associated pixel regions at mutually different angles by similar methods. To increase the optical efficiency and the resolution at the same time, this projection type optical display system divides each image frame into multiple image subframes time-sequentially and periodically switches the angles of incidence of the light rays every time one vertical scan period of the LCD panel passes.
In the projection type optical display system disclosed in PCT International Publication No. WO 01/96932, the white light is split by dichroic mirrors into R, G and B light rays, which are then incident at mutually different angles onto their associated pixel regions of the same LCD panel through a microlens array. Also, data representing a plurality of image subframes are generated from data representing each image frame as an image component. Then, the image subframes are displayed on the LCD panel time-sequentially. Thereafter, by sequentially shifting these image subframes on a projection plane, the same area on the projection plane is sequentially irradiated with multiple light rays that have been modulated by mutually different pixel regions of the LCD panel and that fall within respectively different wavelength ranges (which will be referred to herein as “R, G and B light rays”).
These projection type optical display systems use no color filters, thus achieving high optical efficiency and displaying an image at a high resolution.
In the optical display system disclosed in PCT International Publication No. WO 01/96932, however, a plurality of image subframes, generated from data representing each image frame as an image component, needs to be sequentially shifted on the projection plane. These image subframes are generated for the purpose of displaying an image frame time-sequentially. Accordingly, the interval at which the image subframes are sequentially shifted on the projection plane needs to be much shorter than one period of the image frame.
Also, in the optical display system disclosed in PCT International Publication No. WO 01/96932, this image shifting is carried out by an optical shifter that includes a birefringent element, which changes its refractive index depending on the polarization direction of incoming light, and a polarization direction switching element, which switches the polarization directions of the light to be incident onto the birefringent element. In that case, unless the polarization directions are switched by the polarization direction switching element almost completely synchronously with the switch of image components to be displayed, pixels not be to displayed may be presented. Then, the resolution will decrease, and an unwanted periodic dotted pattern will appear on the projection plane, thus deteriorating the quality of the image projected. Accordingly, in such an optical display system, the polarization directions must be switched at a particularly high response speed.